In recent years, much attention has been paid to Stirling engines because they are energy-saving, environment-friendly, and advantageous from other viewpoints. A Stirling engine is an external combustion engine that realizes the Stirling cycle, a reversible cycle, by the use of an external heat source, and is thus an advantageously energy-saving, low-pollution heat engine as compared with an internal combustion or other engine that requires a highly flammable, ignitable fuel such as gasoline. One widely known form of application of such Stirling engines is Stirling refrigerators.
Conventionally, refrigerators and the like typically adopt a refrigerating cycle based on vapor compression. The vapor compression refrigerating cycle employs a refrigerant such as a CFC (chlorofluorocarbon) as a working medium, and achieves desired refrigerating performance by exploiting the condensation and evaporation of the CFC.
However, CFCs used as refrigerants are chemically highly stable, and are believed to reach the stratosphere and destroy the ozone layer when discharged into the atmosphere. For this reason, in recent years, use and production of particular types of CFC have been increasingly restricted. Under these circumstances, much attention has been paid to a refrigerating cycle based on the reverse Stirling cycle as a replacement for a refrigerating cycle employing a CFC.
The reverse Stirling refrigerating cycle employs helium gas, hydrogen gas, nitrogen gas, or the like as a working medium, and thus has no bad effects on the global environment. Stirling refrigerators exploiting the reverse Stirling refrigerating cycle are known to be compact refrigerators that produce cryogenic low temperature.
A Stirling refrigerator is composed of a combination of a compressor that compresses a refrigerant gas used as a working medium and an expander that expands the refrigerant gas expelled from the compressor. The compressor compresses the refrigerant gas repeatedly in such a way that its pressure varies with time describing, for example, a sine wave. On the other hand, the expander is provided with a cylinder with one end closed, a displacer fitted inside the cylinder so as to reciprocate along its axis and divide the space inside it into an expansion chamber, located in the tip-end side thereof, and a working chamber, located in the base-end side thereof, and a resonant spring that elastically supports the reciprocating movement of the displacer.
The working chamber is connected to the compressor, and the pressure of the refrigerant gas from the compressor causes the displacer to reciprocate and thereby expand the refrigerant gas, producing low temperature in a cooling portion at the tip of the cylinder. This type of Stirling refrigerator is generally called a free-piston-type Stirling refrigerator, of which an increasingly widely used type is one having a piston and a displacer fitted coaxially inside a single cylinder.
In general, the piston is driven with a linear motor. By controlling the voltage with which the linear motor is driven, it is possible to control the stroke over which the piston reciprocates and thereby control refrigerating performance. Specifically, reducing the voltage with which the linear motor is driven results in the piston reciprocating over a shorter stroke and thus in lower refrigerating performance; increasing the voltage with which the linear motor is driven results in the piston reciprocating over a longer stroke, and thus in higher refrigerating performance.
To exploit this relationship, as disclosed in Japanese Patent Application Laid-Open No. H2-217757, it is customary to provide one linear motor to drive the piston and another to drive the displacer, and measure the displacements of the piston and the displacer individually in order to control the currents fed to the linear motors in such a way that the neutral positions of the piston and the displacer are kept in fixed positions.
As disclosed in Japanese Patent Application Laid-Open No. H11-304270, it is also conventionally known to find the stroke of the piston on the basis of the power fed to a driver coil and correct for the offset present in the voltage on the basis of the stroke in order to keep the top dead center of the piston in a fixed position and thereby keep the dead volume of the compression space constant.
However, in the conventional Stirling refrigerators described above, when, at the start of operation, the cold-side temperature is close to room temperature, the internal gas pressure has not yet reached that for steady-state operation, and therefore, if the voltage for steady-state operation is applied to the linear motor, there is the danger of the piston and the displacer colliding with each other. The collision occurs in different manners depending on the structure of the Stirling refrigerator in question. Typically, the displacer collides with the closed end of the cylinder, or the resonant spring fitted to the displacer is compressed to the point of being destroyed. Where the piston and the displacer are fitted coaxially, they may go out of phase, colliding with each other.
The collision is likely to occur also when the refrigeration load so varies as to bring the piston and the displacer out of phase, or when there occurs a variation in an external factor (for example, in the supply voltage to the Stirling refrigerator or in the ambient temperature) while the maximum refrigerating performance is being brought out, or owing to an internal factor (for example, an individual variation such as an assembly error or machining error) of the Stirling refrigerator itself. To avoid the danger of collision, the voltage with which the linear motor is driven needs to be set lower than the ideal voltage, and this makes it impossible to bring out the maximum refrigerating performance of the Stirling refrigerator.
While the Stirling refrigerator is operating, if its cooling portion or heat-rejecting portion is cooled or heated abnormally for some cause, or if the temperature around the Stirling refrigerator varies abruptly, there may occur a variation in the vibration of the balance mass fitted to the body of the Stirling refrigerator to suppress its vibration, increasing the amplitude of the vibration. A variation in the vibration of the balance mass results also from an abrupt variation in the gas balance inside the cylinder, or from deviation of the resonance frequencies of internal components from one another. An increase in the vibration of the balance mass leads to an increase in the noise produced by the Stirling refrigerator and to abnormal vibration, even to collision between internal components, resulting in their destruction.